The electric machines which are configured as traction motors in electric vehicles are as a rule heated to a very pronounced extent during operation. Here, the maximum permissible heating has a considerable influence on the performance limit of the electric machine.
Liquid cooling is therefore frequently used. Here, the rotors are usually cooled with water, since water can absorb and discharge the heat particularly satisfactorily. It is disadvantageous, however, that the rotor often has to be sealed in a very complicated manner, since the water is as a rule pumped through the rotor shaft under pressure. In addition, the seal usually causes a very great power loss and frequently also requires a large amount of installation space.
The use of transmission oil as coolant has therefore been disclosed in the prior art. For example, EP 0 660 492 B1, which is incorporated by reference herein, describes an electric machine having a rotor which is cooled by way of transmission oil. The transmission oil is pumped into a central bore of the rotor shaft through a pipe which is adjoined by a pump. The oil flows back into the gearbox again through an oil outlet opening which is configured in the rotor shaft.
It is a disadvantage of cooling of this type, however; that the transmission oil as a rule experiences pronounced ageing above 120° C. In electric machines and, in particular; in traction motors of electric vehicles, however, the rotor shaft can experience temperatures of 160° C. or more. The transmission oil can therefore deteriorate in a particularly pronounced and/or rapid manner in the case of cooling of this type.